The present invention relates to the general field of electrohydraulic systems, and more particularly it relates to a servovalve for regulating flow rate and used in particular in an aircraft fuel injection circuit.
Conventionally, a servovalve comprises an electric motor, e.g. a torque motor, and a hydraulic distributor valve whose flow rate is controlled to be proportional to the control current applied to the electric motor. It is used in systems that are servo-controlled in position, speed, or force, so as to provide control that is fast and accurate at high levels of power.
In the aviation and aerospace fields where it is becoming more and more commonplace to use computers and electrical controls, servovalves are naturally applied to defrosting or cooling circuits, to piloting compressors, or to adjusting outlet nozzles, or indeed to circuits for injecting fuel, to mention only a few particular examples relating to aeroengines. At present, with that type of servovalve, there can be seen a need to xe2x80x9cfreezexe2x80x9d the position of controlled members in the event of an electrical failure in the aircraft control computer, so that after the breakdown has been found and corrected, said members remain in exactly the same state as they were before the breakdown.
xe2x80x9cFail-freezexe2x80x9d valves that remember their position are well-known to the person skilled in the art. They enable a receiver member associated with the valve to be frozen in a determined position. FIG. 8 shows an example of such a fail-freeze valve 1 associated with an electrohydraulic servovalve 2 for controlling a measurement device 3. The servovalve operates as a conventional three-port servovalve (high pressure (HP) feed 4, return 5, and load 6) together with its electric motor 7 and its hydraulic distributor valve 8 controlled by said motor and supplying a control pressure (or load pressure) for the measurement device as taken from a high pressure feed, the fail-freeze valve interposed between the servovalve and the measurement device being inactive in normal operation. In contrast, in the event of an electrical failure, the servovalve 2 actuated in the opposite direction will, via a fourth port 9, cause said position memory valve 1 to be moved immediately (position shown in FIG. 8), so as to isolate the measurement device 3 which is thus frozen in the position it occupied prior to the electrical breakdown.
Unfortunately, the above structure presents certain drawbacks. Firstly it requires an additional switching stage (also referred to as a third slide), which gives rise to problems of bulk, in particular for onboard apparatuses. Furthermore, the position-freezing action of this structure is exerted only on a single control pressure, which puts a limit on the types of receiver member to be controlled. Finally, during the transient stage between the normal positions for the slides 1 and 2, and the positions corresponding to the slides being frozen, displacement of the slide 1 (to the right in the figure) pushes the slide 3 (to the left) by an amount that corresponds to the volume moved by the slide 1 (the volume common to the chambers of the slides 1 and 3 being incompressible). This movement, even if small, can be harmful in certain applications.
The present invention thus seeks to provide an electrohydraulic device that mitigates the drawbacks of the prior art. An object of the invention is to provide such a device that is simple in structure and particularly compact.
These objects are achieved by a servovalve integrating a fail-freeze function and comprising an electric motor and a distributor valve controlled by said electric motor, said distributor valve having a hydraulic slide which can move linearly inside a cylinder under drive from pressure unbalance created at the two ends of said slide by varying a controlled current for said electric motor, said hydraulic slide comprising a central rod having blocks mounted thereon for co-operating with communication orifices of said distributor valve, and said blocks co-operating with one another and with said ends of the said hydraulic slide to define annular chambers, said communication orifices including at least one high pressure feed orifice, at least one exhaust orifice, and at least two load orifices connected to a receiver member to be controlled, and said annular chambers comprising two pilot chambers, at least two high pressure chambers, at least one low pressure chamber and at least two load chambers, the servovalve further comprising, pierced in said central rod, two load channels for putting each of said load chambers into communication with an immediately adjacent annular chamber so as to ensure that the same pressure is applied on both sides of the blocks separating these two chambers, and wherein, in a predetermined safe position (known as the xe2x80x9cfail-freezexe2x80x9d position) in which said blocks close said load orifices with clearance, the leaks through said load orifices that result from said clearance are drained at a determined pressure (preferably an exhaust low pressure).
Thus, with this particular structural implementation of the distributor valve of a servovalve, it is possible not only to freeze the position of the receiver member controlled by said servovalve, but also and above all significantly to reduce and control leaks and to define the direction in which said control receiver member will drift.
Preferably, the blocks closing the load orifices in said fail-freeze position are mounted with considerable overlap relative to said load orifices. Advantageously, said overlap lies in the range 1 millimeter (mm) to 5 mm.
In a preferred embodiment, the servovalve comprises a central rod provided with six blocks forming seven annular chambers including two pilot chambers situated at the two ends of the distributor valve and five communication orifices in addition to pilot orifices opening out into said pilot chambers. In a variant, the block closing one of the load orifices has two annular drain grooves at its periphery which communicate with the low pressure chamber via a third load channel pierced in the rod.
In another embodiment, the servovalve comprises a central rod provided with eight blocks forming nine annular chambers including two pilot chambers at the two ends of the distributor valve, and seven communication orifices in addition to pilot orifices opening out into said pilot chambers.